The present invention relates to the production of refractory metal alloy coatings on substrates by the chemical vapor deposition (CVD) technique. The invention is particularly directed to the production of such alloy coatings on gun bores.
Gun bores erosion is a serious problem. During the firing of a large number of ammunition rounds, combustion of the predominantly nitrocellulose propellants produces end products of a reducing nature. The high temperatures and pressures encountered during firing procedures cause deterioration of the substrate and erosion of the gun barrel. As a result of the material erosion in the bore, propellant combustion gases escape between the projectile and the bore surface. The gas blowby further increases projectile and barrel erosion and transports eroded particles along the bore where they are redeposited. This action can result in a decrease in projectile muzzle velocity, reduction in firing accuracy and reduction in the effective range of the gun.
Chromium electrolytic plating is at present the most economical method of providing a wear resistant and thermal barrier against the severe conditions encountered in the gun tube. However, the porosity and inherent crack network found in the chrome plate often leads to shearing and flaking in the early rounds of firing, and a progressive softening of the plate occurs with continued firing, thereby decreasing the wear resistance of the plate.
Refractory metals and alloys have been studied as materials for increasing gun tube service life relative to chromium plated steel. Co-extruded gun tubes with tantalum-tungsten liners have been successful in this regard. However, the high cost of the co-extrusion process has stimulated a search for more economical coating methods to provide such refractory alloy surfaces on gun bores.
Since a tantalum-tungsten alloy cannot be suitably plated by conventional methods, the chemical vapor deposition method has been found to be suitable. In the past, conventional chemical vapor deposition practice has utilized metal halides, produced from separate sources and subsequently mixed, for depositing refractory metal alloys. For example, tantalum metal was reacted directly with chlorine gas to form TaCl.sub.5. Similarly, tungsten metal was reacted directly with chlorine gas to form WCl.sub.6. The TaCl.sub.5 and WCl.sub.6 were then mixed with hydrogen and resulting mixture was passed over the heated article to be coated, whereby deposition of Ta-W alloy took place. Ternary alloys were also deposited in similar manner.